System including space discharge device



June 21, 1932.

Original Filed Feb. 7, 1924 2 Sheets-Sheet l lNVENTUR A. A. flaw/440 BY June 1932- A. A. OSWALD SYSTEM INCLUDING SPACE DISCHARGE DEVICE Original Filed Feb. 7, 1924 2 Sheets-Sheet 2 J/VI/ENTUR A. A. 0swAL0 Reissued June 21, 1932 UNITED STATES PATENT OFFICE ARTHUR A. OSWALD, OF EAST ORANGE, NEW JERSEY, ASSIGNOR, BY MESNE ASSIGN- MENTS, TO AMERICAN TELEPHONE & TELEGRAPH COMPANY, A CORPORATION OF NEW YORK SYSTEM INCLUDING SPACE DISCHARGE DEVICE Original No. 1,665,674, dated April 10, 1928, Serial No. 691,173, filed. February 7, 1924. Application for reissue filed October 25, 1928. Serial No. 315,055

This invention relates to systems including three electrodespace discharge devices and more particularly to an arrangement adapted to compensate the effect of alternating current used for heating the cathodes of the devices and also to provide a circuit, including a space discharge device, for selectively amplifying oscillating current whichmaybe given a relatively high degree of amplification without causing the production of undesired oscillations or singing in the amplifier circuits.

As is well known when alternating current is used to heat the cathode of a space discharge device the number of electrons and consequently the amplitude of the current flowing across the space within the device will vary in synchronism with the flucreference to the potentials effective in causing flow of space current is secured.

When a three electrodespace discharge device is included in a circuit for selectivelyamplifying current of' a single frequency the difliculty arises that the circuit has a strong tendency to sing and hence the amount or degree'of amplification possible with ag given amplifier is materially reduced.

The present invention provides an arrangement,'including a space discharge device having its cathode heated by alternating current, which is included in a circuit for selectively amplifying a current of single frequency, having means for compensating for the effect of the alternating heating current and also means for materially reducing the tendency to sing,

' whereby undistorted and relatively highly amplified current may be produced in the output circuit of the amplifier.

4 In accordance with this one aspect of invention an alternating current source supplies heating'current to the cathode of'a three electrode space discharge device the anode of which is connected'to one terminal of the cathode and an alternating voltage of the same frequency but varying inopposite phase relatively to the heating current is impressed upon the impedance control electrode of the device to neutralize the modulating effect of the alternating cathode heating current. The neutralizing voltage wave may be derived from a single phase source which supplies'the heating current or from one or more phases of a plural phase source, one phase of which supplies heating current to the cathode.

In accordance with another aspect of this invention, a selective amplifying system is provided, comprising acircuit including a three element space discharge device associated with a tuned circuit having in series an inductance, a capacityand a resistance, the terminals of which are respectively connected to the cathode and impedance control element of the discharge device.

One object of this invention is to provide improved methods of heating filaments of space discharge devices.

Another object is to correct the distorting efl'ect introduced into space discharge devices resulting from heating the cathodes with alternating current.

A feature of the invention relates to heating oneor more filaments of a system of one or more discharge devices by one or more phases of an alternatin current and applying correcting voltages erived from any one or more phases of the source of heating current to an electrode of one or more of the discharge devices. Y

Another feature of the invention is the provision of means for materially increasing the cfliciency' of a selective amplifying system.

Still another feature is the provision of means whereby a space discharge device having relatively high amplification may be operated without causing singing in the amplifier circuit.

transmitting system includinga space discharge oscillator with an amplifying system embodyin the features referred to above, whereby t e antennacircuit may be supplied with an undistorted modulated carrier wave the frequency of which is substantially constant and independent of impedance variations in the antenna circuit.

For a complete understanding of this in.- vention reference may be made to the following description read in conjunction with the attached drawings in which Fig. 1 shows a circuit arrangement embodyin the invention; Fig. 2 shows a slightly 'di erent circuit arrangement including the invention, and Fig. 3 illustrates the invention applied to a transmitting apparatus for a radio telephone system.

Referring to Fig. 1, there is shown a space discharge oscillator 1, similar to that disclosed in U. S. Patent No. 1,356,753 October 26, 1920 to R. V. L. Hartley, having the inductance coil 2 of its frequency determining circuit inductivel associated with a coil 3 serially connecte with a condenser 4, and resistance 5. The series connected coil 3, condenser 4 and resistance 5 constitute a tuned circuit resonant to the frequency of the current supplied by the source 1. A space discharge device 6'has an input circuit extending from its grid 7 through the tuned circuit and condenser 8 to one terminal 9 of its cathode 10. The drop of potential across the resistance 5 is impressed upon the grid and cathodeof the device 6 to control the flow of space current in synchronism with the current supplied by the source 1..

A polarizing potential is impressed upon the grid 7 by the source 11 through a potentiometer 12. The circuit of the polarizing tential may be traced from the adjusta le contact of the potentiometer the secondary winding of the transformer 13, resistance 5, grid 7, across the space within the device 6 to the cathode 10, terminal 9 to the potentiometer resistance which is connected across the source 11.

Heating current is supplied to the cathode by an alternating current source 14 through a. transformer 15. Theflow of alternating current through the cathode 10 will cause the rate of electron emission to yary synchronously therewith and hence the current flow across space within the device 6 under control of the source 1 will be subject to fluctuations due to the heating current supplied by the source 14.

Space current is supplied to the device 6 by the source 16, through a circuit extending from the positive terminal of the source 16,'through the coil 17, anode18, space within the device to the'cathode 10 and its terminal 9, to the negative terminal of the source 16. i

the device 6 may the circuit.

The alternatingxpurrent output circuit of traced from the cathode 10 across the space within the device to the anode 18, coil 17 and condenser 20 to the terminal 9 of the cathode.

In order that the current present in the output circuit of the device 6 may be a faithful replica of the current supplied by the source 1 it is desirable to compensate or neutralize the fluctuations due to the heating current.

Connected to the source 14 is a phase and amplitude adjusting device 21 comprising a condenser 22, a resistance 23, and a variable inductance or variometer 24. By varying the condenser 22 and inductance 24 the phase of the current traversing the circuit 2223 24 may be shifted relatively to that supplied by the source 14 and by adjusting the contact 25 relatively to the resistance 23 the amplitude of the current supplied to the primary winding of the transformer 13 may be regulated. When the elements of the device 21 are properly.adjustedthere is produced in the secondary winding of the transformer 13, included in the input circuit of the device 6, a potential wave varying synchronously with the current of the source 14 and in such, phase relation thereto that when it is impressed upon the control electrode of the device 6, the distorting or modulating effect of the heating current upon the alter nating current flowing through the output circuit of the device/6 is neutralized or compensated.

As shown in this figure, the space discharge device 6 is used to amplify high frequency current and supply the amplified energy to an antenna circuit which includes a coil 26 inductively associated with the coil 17 in the alternating current output path of the device 6.

In systems for amplifying high frequency current including well known space discharge devices the circuits of which are provided with circuits resonant to the high frequency, a difliculty is presented by the tendency of the amplifier circuits to produce sustained oscillations due to feeding backsonie of the amplified energy to the input side of The amplified energy may be fed back by any kind of coupling relation between the output and input circuits associated with the amplifier. A common form of coupling, and one which is particularly diflicult to compensate for, is that due to the internal capacity of the amplifying devices. If the tendency to develop sustained oscillations is very marked, it is impossible to realize a high degree of amplification since the amplifier must be operated'to give a smaller amplification than that at which the sustained oscillations occur.

It has been found that the production of energy fed back to the tunedcircuit can be prevented by including a resistance, such as 5, of proper value in the tuned circuit and applying the alternating drop of potential across the resistance to the grid 7 of the amplifier. The introduction of the resistance 5 into the tuned circuit renders these circuits selective of a band of frequencies including the high frequency supplied by the source 1. However, lack of sharp selectivity does not materially affect the operation of the amplifiers provided the value of the resistance is so chosen that the resonant circuits suppress current components harmonically related to the carrier frequency, which result from the distorting action of the amplifying devices, and also to limit the amplitude of the energy of the selected frequency to a value such that the production of undesired 'osc'illations is prevented.

As is well known the frequency of the current produced by a space discharge oscillator is susceptible to variations due to impedance changes in the circuits external to the oscillator and hence, if a device of this type is coupled to an antenna circuit by space discharge devices, which inherently possess capacity couplings for high frequencies any variation in the impedance of the antenna circuit will react upon and vary the frequency of the current produced by the oscillator.

However, with the arrangement described above, by using a loose coupling between the source 1 and the input circuit of the amplifier 6, it is possible to supply to the antenna circuit relatively highly amplified energy of substantially constant frequency which is independent of variations in the impedance of the antenna circuit.

Fig. 2 illustrates a transmitting system similar to that described above in which the same reference characters identify the same elements.

In this arrangementthe device 21 which serves for supplying a compensating potential wave of proper phase and amplitude and the potentiometer 12. for supplying the polarizing potential to the grid 7 are included in the lead connecting the cathode terminal 9 to the tuned circuit 3+5. The device 21 is connected to the same source 14 through a transformer 27 The condenser ,17, anode 18, across the space within the .devices to the cathode 10, terminal 9, the

secondary winding of the transformer 27'.

to the negative terminal of source 16.

The alternating output circuit extends transformer 27 is applied to both the grid and plate circuits of the amplifier 6. The amplitude of the voltage supplied to the condenser 22 by the transformer 27 should be so chosen relatively to the voltage supplied by the transformer 15 and these two voltages should be in such phaserelation, as to compensate-the effect due to the alternating heating current supplied to the cathode.

An adjustable condenser 60 is connected in shunt to the secondary winding of the transformer 15.

By adjusting the resistance 23, .the con-' denser 60, or both,'the amplitude and phase of the compensating voltage wave applied to the grid and plate electrodes of the device 6 may be accurately regulated.

If the current supplied by the source 1 is of relatively low frequency the by-pass condenser 22 will not be necessary and the condenser 60 may be omitted.

Referring now to Fig. 3, there is shown a transmitter for a radio telephone system in which a space discharge oscillator 1,

similar tothat describedin the patent referred to above, has the inductance coil 2 of its frequency determining circuit inductively associated with a coil 3 to constitute a loose coupling. iThe oscillator is adapted to supply low poycer waves of carrier frequency to, a tuned} circuit comprising in series the coil 3, condenser 4 and resistance 5 the terminals of which are respectively connected to the grid 7 and cathode 10 of a space discharge amplifier. As previously described the drop of potential across the resistance'5 serves to control the flow of space current within the amplifier.

A polarizing potential is impressed upon the grid 7 by the source 11, through a potentiometer which is connected by a conductor 28 to one terminal'of the source, and to its other terminal by the conductor 29. The circuit of the polarizing potential may be traced from the adjustable contact of the potentiometer through the filtering device 39, resistance 5, grid 7, across the space within the device 6 to the cathode, 10 and through the conductor 29 to the point of beginning. Space current is supplied to the device 6 over the following circuit; from the positive terminal of the source 16, filter 31', conductor 32, resistance 33, coil 17, to the anode 18 across the space within the i device to the cathode and conductor 29 to the negative terminal of the source 16 which is grounded at 34. The alternating current output path extends from the anode 18 over the space path within the device to its cathode 10, condenser 20 and the coil 17 to the anode 18.

Heating current is supplied to the cathodes of the discharge devices by an alternating current source 35, herein shown as a three phase generator.- One phase of the generator is coupled to the conductors 29 and 36 a quency of the carrier wave. The input circuit of the parallel connected devices 38 and 39 extendsjrom their grids to their cathodes and includes the tuned circuit 434445, the right hand conductor of the filter 30 to the conductor 29 Space current is supplied to the parallel connected devices 38 and 39 by the source 16 over the followingcircuit: from the positive terminal of source 16, conductor 32, choke coil 46, .coils 47 and 48, the anodes across the space paths within the devices, to their cathodes and conductor 29 to the negative terminal of the source 16. The alternating current output path extends from the anodes of the devices 38'and 39 across the space paths within the devices to their cathodes, through the condenser 49 and coil 48 to their anodes. 3

' A polarizing potential is im res'sed upon the grids of devices 38 and 39 y the source 11 through the combined potentiometer and filter 30, in a. manner similar to that described above for the device 6.

. Speech currents supplied by the micro-- phone circuit flow through the coil 50 inductively coupled to the coil 51 whichserves to impress them upon the input circuit of the amplifier 40. Space current is supplied to the device 40 from the positive terminal of the source 16 through the filter 31 and conductor 32 across the space within the amplifier to its cathode and conductor 29 to the negative terminal of the source 16. Amplified speech currents flow through the coil 52 and condenser 53 and are impressed upon the common input circuit of the high power amplifiers 41 and 42 which includes the coil 54 inductively associated with the coil 52.

The source 16 supplies spacecurrent to the high power devices 41 and .42 through the filter 31, conductor 32and the choke coil 55 which cooperates with thecoil 46 and the source 16 to constitute a substantially constant current plate supply source for the devices 3839 and 41-42. The high power speech currents are transmitted through the condenser 56 and choke coil 47 to vary the amplitude of the high power carrier wave present in the common alternating current output path ofthe'devices 38-39, whereby modulated carrier waves are supplied by the inductive couplin between the coils 48 and the coil 57 inclu ed in the antenna circuit, for transmission to a distant station. For a further description of the operation of a modulating system similar to that described above, reference may be made to U. S. Patent 1,442,147, January '16, 1923, t R. A. Heising. Coil 47 is used to prevent the flow of high frequency oscillations through the speech frequency circuits.

In a manner similar to that described above for the carrier wave amplifiers, a polarizing potential is impressed upon the control electrodes of the devices 40, 41 and 42 by means of a potentiometer-filter combination 30 from the source 11.

Modulation of the carrier and speech currents by the alternating heating current supplied by one phase of the source 35 to the cathodes of the amplifying devices is prevented by impressing a compensating potential of the correct phase derived from one or more phases of the source 35. By way of example there is shown an arrangement for deriving the compensating voltage wave from the other two phases of the source 35. Connected across each of the other phases of the source 35 is a resistance 58 having an adjustable contact 59 associated with it to constitute a potentiomeer. contacts 59 are connected together by a circuit including a coil 60 inductively associated with a coil 61 included in the input circuit of the amplifier 40. The combination amplified by the tandem connected devices .40 and 4142 to counteract the effect of the Bill" The adjustable i alternating heating current supplied the cathodes of all of the devices included'in the system, viz 6, 38, 39, 40, 41 and 42.

A radio telephone transmitter similar to .that disclosed in Fig. 3 was tested. The

space discharge oscillator generated ,a carvrier current of 2 to 3 watts at 760,000 cycles a small portion of which is amplified by a single stage of amplification to control the ope-ration of two tandem connected stages of amplification eachof. which was provided with a circuit including a series resistance and tuned to 760,000 cycles, voltage upon another electrode of said deand three stages of amplifications were used vice to compensate the efl'ect of the alterto raise the speech currents to high power. nating heating current upon said repeated The energy ultimately delivered to the ancurrent. tenna circuit was 1% kilowatts and the fre- 3. An electrical apparatus comprising a quency of the modulated'wave did not vary Space discharge device having a cathode, more than 50 cycles even when the antenna and a control electrode, means for supplycircuit was opened. ing energy to said control electrode for con- Instead of impressing a derived voltage trolling the space current flowing within wave upon the input circuit of the amplifier Said device, means for supplying alternating 40 to compensate the effect of the alternatcurrent to heat said cathode and means for ing heating current in all of the devices, a impressing upon the control electrode a volt-1 derived wave of the proper amplitud m age wave to compensate the distorting efi'ect beimpressed upon the input circuit of e h of the alternating current supplied to said amplifier, as shown in Fig. 1. hOde- From the preceding description it will be seen that the present invention in one aspect PE dlscharge v e vlng 8: cathode, it provides an arrangement whereby the disg and an f cathode'glld clrcult, tortin d l ti ff t f lt ti cathode-anode circuit connected at one end current used to heat the cathode of the space to T1e te1m1na1 of the ,cathode, meafls 9 discharge device is successfully compensatpp y gy to 531d cathodegrld ed or neutralized, in another aspect means Quit to control flow of p current rier wave frequency f Which i Shh inductance, capacity and resistance connect modified arrafiigements and applied to variare provided for increasing the efliciency of Wlthlll 331d devlce, means pp y g three electrode space discharge devices when ternatlng l heat 831d cflthode, and used for selectively amplifying alternating Q lmpresslng upon 531d Cathodecurrents and in still another aspect contemgrld cu'cult a Voltage WW9 to compensate plates a system in which the. features rethe effect of the'heating current p the f d to above may be d to it a current flowing across the space within said space discharge oscillator to be coupled by devlcethree element space discharge devices to a An electl'lqfll y Compnsmg Space variable impedance transmission circuit so dlschal'ge amphfier havlng a cathode and a that there may be supplied to the tfansmig control electrode, a cathode-control electrode sion circuit an undistorted modulated carfnrcult mcludmg a tuned clrcult havlng an stantially constant and independent of vaed m m Said cathode d COI1tr ol elecriations-in impedance of the transmission 9 bemg FOHHQFtQd to the respective i i mlnals of said resistance, means for sup 1 Although the invention, for the purpose ingalternatmg current to heat said catho e, of completely and clearly disclosing its, na mean? for p y g energy tobeampllfied tum, has been described as applied to radio to said cathode-control element circuit and transmitting systems embodying specific apmeans l' s m p sald 9 paratus, it is to he understood that the prin trol element circuit a voltage wave .to comciples of the different features of this invention may be embodied in many different current P ,the-amphfied rgy- 6. An electrical system comprlsmg a space ous uses and e'nce that this invention is not dlscharge devlce havmg cathode, an anode limited to the use or details described above, f control electrode means for Supply but only by the scope of the attached claim s. mgenergy to control h 0Pemti0n.f a

The invntion claimed device means for supplying alternating cura rent to said cathode, and means for impress- 1. An electrical a aratus com risin space discharge devidie having a pliirality of a voltage Wave. upon the other electrodes electrodes one of which is a cathode, means for s pplying energy to control the Opera- 23 3 alternating current Supplied to said ca 0 e.

pensate the efi'ect of the alternating heating said device for compensating the effect tion of said device, means for supplying alternating current to said cathode and means for impressing. a voltage wave upon another electrode of said device to compensate the distorting effect of the alternating current; supplied to the cathode. K

2. An electrical apparatus comprising a space discharge device having a cathode, means for supplying to said device current 7. In an amplifier system, a space discharge device having a cathode and a control electrode, a resonant input circuit in cluding an inductance, a capacity and a resistance inseries the resistance having its respective terminals connected to the cathode and control electrode of said device.

8. In a system for producing a high to be repeated, means for supplying alterpower carrier wave of suhstantially constant nating current to heat said-cathode, and frequency, the combinatlouof a space dismeans for impressing an alternating current charge osclllator forsupplying a low power 4. An electrical apparatus comprising a r 7 carrier wave, an amplifying system for transforming said low power wave into a wave of high power comprising a plurality of tandem connected circuits each including a three element space discharge device and a circuit tuned to the carrier frequency including in series, inductance, capacity, and a resistance for coupling the discharge device thereto so that the production of undesired oscillations isprevented.

9. In a system for producing a high power carrier waveof substantially constant frequency, ,thecombination of a space discharge oscillator for supplying a low power carrier wave, an amplifying system for transforming said lower power wave into a wave of high power comprising a plurality of tandem-connected circuits each including a three-element space discharge device and a circuit tuned to the carrier frequency, said circuit comprising series connected induc tance, capacity and resistance, the voltage across the resistance serving to control the included discharge device, whereby the production of undesired oscillations is prevented. v

10. A carrier transmitting system, comprising a source of carrier waves, means for coupling said source to a transmission circuit by a circuit including a space discharge device and a tuned circuit including a series connected means for preventing impedance variations in said transmission circuit from reacting upon said source of carrier waves.

11. A carrier transmitting system comprising a source of carrier waves, means for coupling said source to a transmission circuit by a circuit including a space discharge oscillator for supplying high frequency waves, means for coupling said oscillator to a transmission circuit by a circuit including a. space discharge device and a tuned circuit including series connected means for preventing impedance variations in said transmission circuit from reacting upon said oscillator. 1

12. A carrier transmitting system comprising a space discharge'oscillator for supplying high frequency waves, means for coupling said oscillator to a transmission circuit by a circuit including aspace discharge device and having a plurality ofelectrodes one of which is a cathode, and a tuned circuit including series connected means for preventing impedance variations in said transmission circuit from reacting upon said oscillator, means for supplying alternating current to heat said cathode, andmean's for impressing upon another electrode of said discharge device a voltage wave to compensate the effect of the alternating heating current supplied to said cathode.

13. A carrier transmitting system comprising a space discharge oscillator, a space ischarge ampl1fier,means for coupling said oscillator to said amplifier, including a tuned circuit having inductance, capacity and resistance in series, means for connecting the input electrodes of said amplifier to the respective terminals of said resistance,

a transmission circuit, andmeans for sup-- plying amplified oscillations to said transmission circuit. i

14. A carrier transmission system comprising a space discharge oscillator producing carrier currents of low power, means for raising said, low power currents to high power comprising a tuned circuit including in series with the tuning elements a resistance, and a space discharge amplifier having input and output electrodes, said input electrodes connected to the terminals of said re-' sistance, a transmission circuit, and means associated with said output electrodes for same frequency comprising tandem connected circuits, each including a tuned circuit having aresistance and a space discharge amplifier having input and output electrodes withsaid input electrodes connected to the terminals of said resistance; a low power source of modulating current, tandem connected circuits for raising said modulating current tohigh power, each including a space discharge amplifier; means for combining said high power carrier current and modulating current to produce a modulated carrier current, a transmission circuit, and

means included in, a circuit connected to the output electrodes of one of said carrier current amplifiers for supplying said modulated currents to. said transmission circuit.

16. A carrier transmission system, a space discharge oscillator producing low power currents of carrier frequency, means for currents into high power currents of the transforming said low power currents into high power currents of the same frequency comprising tandem connected circuits, each including a tuned circuit having a resistance and a space discharge amplifier having input and output electrodes with said input electrodes connected to the terminals of said resistance; a low power source of speech currents, tendem connected circuits for raising said speed currents to high power each including a space discharge amplifier; means for combining said high power carrier-[currents and speech currents to produce a modulated carrier current, a transmission circuit, and means'included in. a circuit connected to the output electrodes of one of saidcarrier -current amplifiers for supplying said speech modulated carrier current to said tr smission circuit.

tions in said transmission circuit from re' acting upon said oscillator; a: source of modulating current, an amplifying system for the modulating current comprising tandem connected circuits each including a space discharge amplifier having a plurality of electrodes, one of which is a cathode, means for supplying alternating current for heating the cathodes of said carrier wave and modulating current amplifiers, means associafiad with a second electrode of one of said modulating currentamplifiers for supplying a controlpotential thereto for compensating the distorting effect of said alternating'heati-ng current, and means for combining said high power carrier wave and said amplified modulating current whereby there is produced an undistorted modulated carrier wave the frequency of which is substantially constant and independent of impedance variations in the transmission circuit.

18. A carrier transmission system, comprising a space discharge oscillator for producing a carrier wave of low power, a transmission circuit, means for coupling said oscillator to said transmission circuit consisting of tandem connected circuits each including a tuned circuit and a space dischar e amplifier having input electrodes one of w ich is a cathode, means for connecting an element of said tuned circuit to said in-- put electrodes to prevent distortion of the carrier wave and also to prevent impedance variations in said transmission circuit from reacting upon said oscillator; a source of speech currents, an amplifying system for the speech currents comprising tandem connected circuits, each including a space discharge amplifier having a plurality of electrodes one of which is a cathode, means for supplying alternating current for heating the cathodes of said carrier wave and speech current amplifiers, means associated with a second electrode of one of said speech current amplifiers for supplying a control potential thereto for compensating the distor-ting effect of said alternating heating current, and means for combining said hlgh power carrier wave and said amplified speech currents, whereby there is produced an undistorted modulated carrier wave the frequency of which is substantially constant and independent ofimpedance variations in the transmission c rcuit.

19. In combination, a thermionic device I havinga filament adapted to be heated for causing it to emit electrons, and a plate electrode, a circuit connecting said electrodes, a source of fluctuating potential connected to the filament for heating it, means operating in response to the size and direction of the variations in current due to variations in said potential to affect the space between the filament and plate, in such a sense as to tend to maintain the space'current constant in spite of said variations, andv means cooperating with said affecting means for allowing 1mpedance variations in the path to vary the current in said path.

20. In combination, a thermionic device having a filament adapted to be heated for causing it to emit electrons, and a plate electrode, a circuit connecting said electrode, a source of fluctuating potential connected to the filament. for-heating it, means afiecting the impedance of this circuit in response to each variation of current due to variations in said potential, in such a sense as to tend to maintain the space current constant in spite of the fluctuations, other means causing additional variation in impedance in the space between the electrodes, and means 00- operating with said afiecting means and operating in response to such variations in impedance for allowing the impedance variations to affect the current flow in said circuit.

21. The method of eliminating feed frequency tones and harmonics thereof in modulating systems utilizing alternating or rectifled currents which consists in, deriving from such currents another current and supplying such current to the modulating current in a .manner to neutralize the feed frequency current, introducing such current into the circuit traversed bv the modulating current and adjusting the phase of the current so introduced in a manner to neutralize the undesired frequencies.

f 23. In radio signaling apparatus, the combin ati on of a transmission conductor, an os'- cillator for energizing said conductor, means for modulating the output of said oscillator, a source of power for energizing said oscillator. means associated therewith for producing an alternating current of any desired phase relation -with respect to the current delivered by said source, 'means for combining such current with the modulating current, and means for varying the intensity of such current.

24. In radio signaling apparatus, the combination of a transmission conductor, an oscillator for energizing said conductor, fneans for modulatin the output of saidoscillator, a source for eeding said oscillator, phase shifting means for producing a current having a desired phase relation with the oscillator current, and variable coupling means for combining such current with the modulating current.

25. In a radio system a transmission conductor, means for producing oscillations in said conductor, means for modulating said oscillations with speech currents, means for producing a current'in any desired phase relationship to said oscillations, and means for combining said last named current with said modulating speech currents.

26. In a radio system an oscillator, a source of low frequency alternating current for feeding said oscillator, means for modulating the output of said oscillator, and means connected with said source for causing cur rent flow in said modulating means of the same frequency as said source, and in any desired phase relationship. i r g5 27. In a radio system an oscillator, a ,source of low frequencyialternating current for feeding said oscillator, means for modulating the output of said oscillator, and means for causing current flow in said modulator of the-same frequency as said source, said last named means including means for varying the intensity of said current, and means for varying the phase of said current.

In witness whereof, I hereunto subscribe my name this 17th day of October, 1928. ARTHUR A. OSWALD. 

